Led illuminated lamp with thermoelectric heat management

ABSTRACT

The present invention relates to Light Emitting Diode (LED) based lamps utilising thermoelectric modules improving the efficiency of the lamps. The present invention provides in a first aspect a light illuminating device that comprises at least one light emitting diode (LED), at least one thermoelectric module (TEM) having a first surface which is thermally connected to the LED, a heat sink thermally connected to a a second surface of the at least one TEM, a thermally insulating cover creating a chamber substantially insulating the LED from ambient air. The LED may be of any conventional type, the invention however is particularly useful for devices using hi-flux LEDs, including traffic lights, illuminated roadway and/or emergency signs, airport runway lights and such.

FIELD OF INVENTION

The present invention relates to LED-based lamps utilisingthermoelectric modules Improving the efficiency of the lamps.

BACKGROUND OF THE INVENTION

Practical design and application of Light Emitting Diodes (LED) typedevices for use in Area Lighting and like schemes are limited by thermalenergy-management issues. LED device manufacturers have generally beenaiming at developing LED devices that provide greater light outputwithout significant increase In size of the device. This accentuates theproblem of heat management; the energy efficiency of LEDs is relativelylow, such that only a portion of the consumed energy is converted tolight, while the bulk of the energy is converted into heat. Therefore,by by producing more light intensive LEDs more thermal energy isproduced in the same unit volume of the device.

It is known that LEDs exhibit negative temperature coefficient aspects,i.e. at fixed power Input, as the device's operating heat rises, thedevice's light output decreases. The relationship between LED decreasein light output due to increased operation temperature can be expressedapproximately as a negative linear relationship between the percentagelight output and degree C. Increase in temperature. That is, as the LEDdevice's operating temperature increases one ° C. it can by approximatedthat the device will lose about one percent of its light output.

Attempts have been made in the prior art to solve the negativetemperature coefficient issues. As an example, in LED highway trafficsignal devices housings with ventilation configurations, both of passive(convection-type) and active (fan-driven-type) have been provided toprevent the LED-s from overheating. Present art LED traffic signaldevices also address the inherent negative temperature coefficientnature via the electrical power supply. These approaches either increasepower to the device to compensate for light output loss or address theform of the provided electrical power such as sine vs. square wave in anattempt to moderate the production of by-product heat, i.e. waste heat.

There is a long-felt need for LED devices of long service life and highelectric power-to-light efficiency.

Solid state thermoelectric modules (TEM) also referred to asthermoelectric coolers (TEC) or heat pumps have been used in variousapplications since the Introduction of semiconductor thermocouplematerials. Such devices convert electrical energy into a temperaturegradient, known as the “Peltier” effect or convert thermal energy from atemperature gradient Into electrical energy. By applying a currentthrough a TEM a temperature gradient is created and heat is transferredfrom one side, the “cold” side of the TEM to the other side, the “hot”side.

TEMs have been considered unsuitable in the art for cooling LED lightingdevices as they have been ruled out for Insufficient efficiency; thatis, if configured and operated with conventional settings the energycost of operating a TEM for cooling an LED device is more than theenergy gained in operating the LED at a reduced temperature.

SUMMARY OF INVENTION

The present Inventors have now found that TEMs can surprisingly be usedto cool and enhance the light output of LED lighting fixtures andparticularly to maintain optimal light output from LED lightingfixtures.

The Inventors have analyzed the behaviour of conventional Hi-flux LEDsand such as used in traffic signals and area lighting devices and TEMsand developed mathematical models for that are used to optimize theperformance of the TEM-cooled LED devices of the present invention.

Thermal Properties of High Flux LEDs

As seen In FIG. 2, the light emission from conventional Hi-flux 1 W LEDsas a function of temperature can be approximated as exponentialfunctions By measuring the light output of two LEDs, amber and red,empirical functions were derived defining the temperature-dependentbehavior of the diodes. The functions define relative output (η)normalized around 25° C. and will be valid for temperatures from about−20° C. up to about 110° C. The functions for the amber diode (η_(A))and red diode (η_(R)) are shown as Equation Ia and Ib respectively.$\begin{matrix}{{\eta_{A}(T)} = {{{{\mathbb{e}}^{(\frac{25.6 - T}{61.6{{^\circ}C}})} \cdot {\eta_{A}\left( {25.6{{^\circ}C}} \right)}}\quad{\eta_{R}(T)}} = {{\mathbb{e}}^{(\frac{24.6 - T}{95.4{{^\circ}C}})} \cdot {\eta_{R}\left( {24.6{{^\circ}C}} \right)}}}} & {{{{Eq}.\quad{Ia}}\&}\quad{Ib}}\end{matrix}$

A LED reference temperature can be defined as “T₀=298.75K” for amber,but “T₀=297.75K” for red. Also, a LED characteristic temperature (T_(L))for a specific LED can be defined and is “T_(L)=61.6K” for amber, but“T_(L)=94.5K” for a red LED.

A low temperature operation of the LED Is beneficial due to reducedwaste heat generation. It is, however, not possible to “save energy” bycooling, In fact the “saved” energy is converted to light. Thus, bycooling the LED, less energy (e.g., fewer diodes) can be used to producethe same amount of light.

COP or the coefficient of performance Is the amount of heat carriedaround in a is system, divided by the work In doing so. When cooling aLED with a thermoelectric cooler (TEC), the heat pumped will beP_(LED)(1-η_(LED)) and the work by the TEC in doing so will be P_(TEC).$\begin{matrix}{{{COP}\quad(T)} = {\frac{P_{LED}}{P_{TEC}} \cdot \left( {1 - {\eta_{0} \cdot {\mathbb{e}}^{(\frac{T_{0} - T}{T_{L}})}}} \right)}} & {{Eq}.\quad{II}}\end{matrix}$

Here “η₀=P_(PHO)/P_(LED when T equals)” is the LED photon efficiency. Amore efficient LED will generate lesser heat and therefore can use acooling system with a lower COP.

The total power consumed by the LED and TEC system (P_(SYS)) will nowbe: $\begin{matrix}{P_{SYS} = {{P_{LED} + P_{TEC}} = {P_{LED} \cdot \left( {1 + \frac{1 - {\eta_{0} \cdot {\mathbb{e}}^{(\frac{T_{0} - T}{T_{L}})}}}{COP}} \right)}}} & {{Eq}.\quad{III}}\end{matrix}$

The total system power (P_(SYS)) converges to the LED power (P_(LED)) asthe COP of the cooling system approaches infinity.

The Luminal or Photonic power radiated by the LED (P_(PHO)) can berelated to the total system power. This can be expressed as:$\begin{matrix}{P_{PHO} = {P_{SYS} \cdot \left( \frac{\eta_{0}}{{\left( {1 + \frac{1}{COP}} \right) \cdot {\mathbb{e}}^{(\frac{T - T_{0}}{T_{L}})}} - \frac{\eta_{0}}{COP}} \right)}} & {{Eq}.\quad{IV}}\end{matrix}$

To realize a certain performance, let η_(SYS)=P_(PHO)/P_(SYS) signifythe total lamp efficiency with cooling system. Then we can calculate theCOP necessary to obtain the desired performance: $\begin{matrix}{{COP} = \frac{\left( {1 - \eta_{LED}} \right)}{\left( {\frac{\eta_{LED}}{\eta_{SYS}} - 1} \right)}} & {{Eq}.\quad V}\end{matrix}$

As an example if the LED efficiency is 20% and the total efficiency acooled LED device (lamp efficiency) is acceptable as 15%, a COP=2.4 isneeded. Further, if a LAMP efficiency of 18% is required, a COP=7.2 isneeded, which is a rather large value, but if LAMP efficiency isacceptable as 10% we can operate the cooling system at COP=0.8 which isa rather modest value.

In the following T_(W) represents the un-cooled LED temperature andT_(C) represent the cooled LED temperature with T_(W)>T_(C). To benefitfrom cooling, the following inequality must be fulfilled:$\begin{matrix}{\frac{P_{LED}}{P_{SYS}} < \frac{{\mathbb{e}}^{(\frac{T_{W} - T_{o}}{T_{L}})}}{{\left( {1 + \frac{1}{COP}} \right) \cdot {\mathbb{e}}^{(\frac{T_{C} - T_{o}}{T_{L}})}} - \frac{\eta_{0}}{COP}}} & {{Eq}.\quad{VI}}\end{matrix}$

The inequality in Eq. VI can be solved for the COP as a function of thetwo temperatures T_(W) and T_(C), and the single power quota η_(SYS):$\begin{matrix}{{{COP} > \frac{P_{LED} \cdot \left( {1 - {\eta_{0} \cdot {\mathbb{e}}^{(\frac{T_{0} - T_{C}}{T_{L}})}}} \right)}{{P_{SYS} \cdot {\mathbb{e}}^{(\frac{T_{W} - T_{C}}{T_{L}})}} - P_{LED}}} = \frac{\eta_{SYS}\left( {1 - {\eta_{0} \cdot {\mathbb{e}}^{(\frac{T_{0} - T_{C}}{T_{L}})}}} \right)}{{\mathbb{e}}^{(\frac{T_{W} - T_{C}}{T_{L}})} - \eta_{SYS}}} & {{Eq}.\quad{VII}}\end{matrix}$High COP Operation of Industry Standard Thermoelectric Coolers

The thermoelectric material properties and geometry and electriccurrent, can be represented by two temperatures, “T_(I)=Ri/α” signifyingthe normalized current driven through the module, and “T_(G)=KR/α²”signifying the quality of the thermoelectric material. Here a representthe so-called Seebeck Coefficient, measured in Volts/Kelvin, Krepresents the integrated thermal conductance of the TEM, measured inWatts/Kelvin, and (i) represent the electric current driven through theTEM, measured in Amperes, and (R) represent the DC resistance, measuredin Ohms. The Cooling Power of the TEM is expressed as: $\begin{matrix}{\overset{.}{Q_{C}} = {\frac{K}{T_{G}} \cdot \left( {{T_{i} \cdot T_{C}} - {{T_{G} \cdot \Delta}\quad T} - {\frac{1}{2} \cdot T_{i}^{2}}} \right)}} & {{Eq}.\quad{VIII}}\end{matrix}$

Here T_(H) represent the hot side temperature and T_(C) represent thecold side temperature with T_(H)>T_(C), and ΔT=T_(H)−T_(C). The rate ofwork done in driving the current through the module is expressed as:$\begin{matrix}{\overset{.}{W} = {\frac{K}{T_{G}} \cdot \left( {T_{i} + {\Delta\quad T}} \right) \cdot T_{i}}} & {{Eq}.\quad{IX}}\end{matrix}$

The rate of heat rejected out of the hot side of the thermoelectricmodule is calculated as: $\begin{matrix}{\overset{.}{Q_{H}} = {\frac{K}{T_{G}} \cdot \left( {{T_{i} \cdot T_{H}} - {{T_{G} \cdot \Delta}\quad T} + {\frac{1}{2} \cdot T_{i}^{2}}} \right)}} & {{Eq}.\quad X}\end{matrix}$

The so-called “coefficient of performance”, COP, is defined as thecooling power divided by the rate of work done by the thermoelectricpower supply: $\begin{matrix}{{COP} = {\frac{{\overset{.}{Q}}_{C}}{\overset{.}{W}} = \frac{{T_{C} \cdot T_{1}} - {{T_{G} \cdot \Delta}\quad T} - {\frac{1}{2} \cdot T_{i}^{2}}}{\left( {{\Delta\quad T} + T_{i}} \right) \cdot T_{i}}}} & {{Eq}.\quad{XI}}\end{matrix}$

The electric current that maximizes the COP can be obtained analyticallyfrom equation XI by differentiation and equating the result to zero:$\begin{matrix}{i_{COPMAX} = {\frac{{K \cdot \Delta}\quad T}{\alpha \cdot \overset{\_}{T}} \cdot \left( {1 + \sqrt{1 + \frac{\quad\overset{\_}{T}}{T_{G}}}} \right)}} & {{Eq}.\quad{XII}}\end{matrix}$

T(bar) is the average temperature equal to (T_(H)+T_(C))/2, usually inthe center of the thermoelectric material along the thermal gradient.

Heat Sink Parameters for a LED-TEC System

The heat sink requirements can be assessed by sufficient accuracy with asingle physical heat transfer coefficient (h) measured in [W/m²K] andits effective area (A) measured in [m²]. To balance the heat sinkoperating in an ambient temperature of (T_(amb)) with the hot side ofthe TEC at temperature (T_(H)), we have: $\begin{matrix}\begin{matrix}{\quad{Q_{\quad H}\quad = \quad{A \cdot h \cdot \left( \quad{T_{\quad H}\quad - \quad T_{\quad{amb}}} \right)}}} \\{\quad{= \quad{\frac{K}{T_{G}} \cdot \left( {{T_{i} \cdot T_{H}} - {{T_{G} \cdot \Delta}\quad T} + {\frac{1}{2} \cdot T_{i}^{2}}} \right)}}}\end{matrix} & {{Eq}.\quad{XIII}}\end{matrix}$

By defining a characteristic area for the TEC-heat sink combination as“A₀=K/h” measured in [m²], we can simply write: $\begin{matrix}{\frac{{T_{G} \cdot T_{C}} - {\left( {T_{G} - T_{i}} \right) \cdot T_{H}} + {\frac{1}{2} \cdot T_{i}^{2}}}{T_{G} \cdot \left( {T_{H} - T_{amb}} \right)} = \frac{A}{A_{0}}} & {{Eq}.\quad{XIV}}\end{matrix}$

Now it is possible to eliminate the hot side temperature (T_(H)) toreduce the degrees of freedome for the LED-TEC design: $\begin{matrix}{T_{H} = \frac{{A_{0} \cdot T_{G} \cdot T_{C}} + {A \cdot T_{G} \cdot T_{amb}} + {{\frac{1}{2} \cdot A_{0}}T_{i}^{2}}}{{A_{0} \cdot T_{G}} + {A \cdot T_{G}} - {A_{0} \cdot T_{i}}}} & {{Eq}.\quad{XV}}\end{matrix}$

To take an example, for the specified TEC with “K=0.6 W/K”, and usingthe empirical value “h=14 W/m²K”, the characteristic area for such asystem is “A₀=K/h=400 cm²” and corresponds to 20 cm by 20 cm flatsurface.

Nonlinear Equations of the Combined LED-TEC-HEATSINK System:

Equation V and Equation XI both express the COP for the LED and TECrespectively. When made equal they give nonlinear and transcendentalequations In the temperature and efficiency variables of the system.$\begin{matrix}{{{\left( {{T_{i} \cdot T_{H}} - {{T_{G} \cdot \Delta}\quad T} + {\frac{1}{2} \cdot T_{i}^{2}}} \right) \cdot \eta_{S} \cdot \exp}\quad\left( \frac{T_{C} - T_{0}}{T_{L}} \right)} = {{{\left( {{T_{i} \cdot T_{C}} - {{T_{G} \cdot \Delta}\quad T} - {\frac{1}{2} \cdot T_{i}^{2}}} \right) \cdot \exp}\quad\left( \frac{T_{W} - T_{0}}{T_{L}} \right)} + {T_{i} \cdot \left( {T_{i} + {\Delta\quad T}} \right) \cdot \eta_{0} \cdot \eta_{S}}}} & {{Eq}.\quad{XVI}}\end{matrix}$

It should be noted that the analysis can be simplified by only lookingat the LED which is being cooled without the constraint relating to thenon-cooled diode and setting T_(W), to equal T₀, then the exponential onthe right hand side of the equation would cancel out. This wouldsimplify the subsequent analysis as well.

By introducing the shorthand notation “β_(c)=exp((T_(C)−T₀)/T_(L))” and“β_(W)=exp((T_(W)−T₀)/T_(L))”, Equation XVI can be rewritten as a secondorder algebraic equation in the variable (T_(I)). This will give the TECparameters to satisfy a certain design goal for a specific LED.$\begin{matrix}{{{\frac{1}{2} \cdot T_{i}^{2}} + {\frac{\left( {{T_{H} \cdot \eta_{S} \cdot \beta_{C}} + {T_{C} \cdot \beta_{W}} - {\Delta\quad{T \cdot \eta_{0} \cdot \eta_{S}}}} \right)}{\left( {{\eta_{S} \cdot \beta_{C}} + \beta_{W} - {2 \cdot \eta_{0} \cdot \eta_{S}}} \right)} \cdot T_{i}} - \frac{{T_{G} \cdot \Delta}\quad{T \cdot \left( {{\eta_{S} \cdot \beta_{C}} - \beta_{W}} \right)}}{\left( {{\eta_{S} \cdot \beta_{C}} + \beta_{W} - {2 \cdot \eta_{0} \cdot \eta_{S}}} \right)}} = 0} & {{Eq}.\quad{XVII}}\end{matrix}$

Equation XVII completes the LED-TEC-Heat sink analysis.

The present invention provides in a first aspect a light illuminatingdevice that comprises

-   -   at least one light emitting diode (LED),    -   at least one thermoelectric module (TEM) having a first surface        which is thermally connected to the LED,    -   a heat sink thermally connected to a a second surface of the at        least one TEM,    -   a thermally insulating cover creating a chamber substantially        Insulating the LED from ambient air.

The LED may be of any conventional type, the invention however Isparticularly useful for devices using hi-flux LEDs, including trafficlights, illuminated roadway and/or emergency signs, airport runwaylights, vehicle lights including brake lights. In useful embodiments ofthe invention, the device comprises a plurality of LEDs.

It will be appreciated that the device of Invention is able to producemore light per unit energy consumed, than corresponding LED-based lightswithout cooling, because the additional energy needed to operate the TECIs less than the energy and/or light output gained. Hence, in the mostpreferred embodiments of the Invention, the device is configured suchthat the device produces more Illumination per unit consumed power whenthe TOC Is applied to the TEM, than the illumination produced per unitconsumed power when no TOC is applied to the TEM.

The thermal connection between the LED and TEM can be realized by aninterface of thermally conducting material, e.g. a metal such as copperor aluminium. The surfaces of the TEM are typically referred to as the“hot side” and the “cold side”, where the cold side is the first surfacein contact with the LED and the hot side the second surface in contactwith the heat sink or adjacent TEM optionally connected by a thermallyconducting plate. However, It should be born in mind that thetemperature gradient of the TEM can be reversed by reversing the currentapplied to the TEM.

To realize the desired efficiency that makes TEM cooling worthwhile, theat least one TEM is chosen and configured such that the device can beoperated by running a TEM-operating current (referred to herein as TOC)through the TEM which is substantially less that the maximum operatingcurrent for the TEM (i_(max), typically the maximum operating currentspecified by the TEM manufacturer), preferably the TOC is less thanabout 25% of i_(max), more preferably less than about 20% of thei_(max), and even more preferably less than 15% of the i_(max), such asless than about 12% of the i_(max). Such configuration can surprisinglystill prevent a decrease in light output due to an increase of thetemperature of the LED(s).

In preferred embodiments, the optimal TOC is in the range of 200-600 mA,such as e.g. is in the range of 200-600 mA, or in the range of about250-350 mA. These values would typically apply when using a TEM withi_(max) of about 3.0 A.

It is not necessarily desired to obtain cooling of the LED substantiallybelow the ambient temperature, on the contrary, the Inventors have foundthat the desired efficiency and energy saving/light gain of the presentinvention is obtained by keeping the operating temperature of the LEDclose to or just below the ambient temperature. In some embodiments theoperating temperature of the LED may even be slightly higher than theambient temperature, but importantly, the LED operating temperature isprevented from rising much above ambient temperature, such as would bethe case for an LED-lamp with no cooling. If the ambient temperature is,e.g., about 20-25° C., a non-cooled LED may be expected to warm upduring operation and within a relatively brief period reach an operatingtemperature in the range of about 50-60° C., at which point theillumination of the LED has decreased by about 30-40% or more due to thenegative Illumination-temperature coefficient.

The heat sink is generally of a conventional type, i.e. with a flatsurface that Is in contact with the TEM's hot side, while the other sideof the heat sink has an extensive surface area to efficiently dissipatethe heat to the air in contact with the heat sink.

The desired COP of the TEM of the device of the invention depends on thedesired overall energy efficiency of the device, generally the device Isconfigured and operated such that the COP of the TEM will lie in therange of about 2-6.

From the above discussion and analysis it follows that particularlypreferred embodiments of the invention relate to devices configured suchthat the device produces more illumination per unit consumed power whenthe TOC Is applied to the TEM, than the Illumination produced per unitconsumed power when no TOC is applied to the TEM. For example, if theTOC consumes 30% of the energy consumed by LEDs of a multi-LED lamp, thetotal energy consumption Is 130% when the device is being cooled and100% If the device Is operated with no cooling; if this prevents thediodes from warming up and loosing 50% light output, the number ofdiodes In the lamp can be halved in the cooled lamp to obtain the samelight intensity, reducing the LED energy to 50% and thus the overallenergy consumed is 80%, i.e., a net energy gain of 20% can be obtainedIn this example by cooling the LEDs In accordance with the Invention.

In certain embodiments, the device of the invention comprises aplurality of TEMs. These may arranged side by side, e.g., each arrangedto cool a set of LEDs. Also, TEM may be arranged in a stacked fashion,such that two, three or more TEM form a “sandwich” wherein the TEMclosest to the LEDs has Its hot side thermally connected (eitherdirectly adjacent or connected with a thermally conducting material) tothe cold side of a second TEM, which also may have its hot sideconnected to the cold side of a third TEM and so forth. The layers ofthe stacked TEMs may overlap or bridge two or more TEMs of the nextlayer so as to provide multiple routes for heat transfer. When usingsuch stacked TEM, the heat sink can be seen as comprising thecombination of the additional TEMs, any intermediate heat-conductingplates and the heat sink itself furthest away from the LED in thesandwich of components.

The thermally Insulating cover creating a chamber reduces flow of heatfrom ambient air to the LED, and is important in the situation where theLED is operated at a temperature below ambient temperature. The covercan be transparent so a not to block the light from the LED(s), and maybe e.g. In the form of a lamp lens. However, In some embodiments, theenclosed chamber does not necessarily fully enclose the LED(s), but Itstill functions do reduce potential heat transfer from the environmentto the LED. The chamber can, for example, have an opening for the lensof the LED.

In certain embodiments, the chamber has a higher pressure within thechamber than ambient pressure, during normal operation. This can beuseful to prevent humidity from the air to enter the chamber. In otherembodiments, the chamber may have a reduced pressure as compared to theambient air pressure, and the chamber may even have a partial orsubstantial vacuum.

The device comprises in one embodiment a control unit for controllingand even reversing the TOC, and one or more sensors connected to thecontrol unit for sensing one or more environmental parameters, whereinthe control unit Is configured to adjust the TOC based on parametersmeasured by the one or more sensors.

Such one or more sensors may comprise a temperature sensor for measuringthe in situ temperature surrounding the LED(s), or the temperature ofthe LED Itself, in which case the control unit essentially functions asa thermostat. Additionally, or alternatively, said one or more sensorsmay comprise a sensor for measuring emitted light from the LED(s), suchthat, for example, if light output decreases, the TOC is increased. Thereverse can also be effected, i.e. decreasing the TOC to reducing thelight output. Thus the device would ensure that a stable amount of lightoutput is maintained.

It may In some cases be beneficial to operate the device with pulsedcurrent to the one or more LEDs, e.g. such that current pulses alternatebetween different LEDs of the device.

In certain embodiments the device may be operated with pulsed current tothe one or more TEMs, e.g., such that current pulses alternate betweendifferent TEMs of the device. In further embodiments, It may bebeneficial to operate the device with pulsed current to the one or moreLEDs and/or TEMs, e.g., such that current pulses alternate betweendifferent LEDs and/or TEMs of the device.

A related aspect of the invention provides a light illuminating devicecomprising at least one light emitting diode (LED) and at least onethermoelectric module (TEM) thermally connected to the LED, and a heatsink; wherein the at least one TEM is selected and configured such thatby running a TEM-operating current (TOC) through the TEM, the thermalpower produced by the at least one LED is transferred through the atleast one TEM to the heat sink, thereby maintaining or lowering thetemperature surrounding the LED and enhancing the light output from theLED; the device thus consuming less overall power per amount of emittedlight when the TEM is running as compared to the overall power per sameamount of light when the device is operated without running an operatingcurrent through the TEM.

In a further aspect, the invention provides a method for enhancing theefficiency of an light illuminating device having one or more LEDs as alight source, comprising: providing the device with one or morethermoelectric module(s) (TEM) having a cold surface and a hot surface,such that the cold surface is thermally connected to the LED and the hotsurface is thermally connected to a heat sink; applying a TEM-operatingcurrent (TOC) to the one or more TEMs to create a temperature gradientthrough the TEM; adjusting the TOC such that substantially all of thethermal energy created by the LED(s) when operated is transferred to theheat sink, thereby substantially maintaining the operating temperatureof the LED(s) at ambient temperature or a lower temperature, wherein theTEM is configured and TOC adjusted such that the device consumes lessoverall power per amount of emitted light when the TEM is running ascompared to the overall power per same amount of light when the deviceis operated without applying a TOC to the TEM.

Preferably, the device is configured and operated such as describedabove, e.g. by applying a TOC that is less than 20% of the maximumoperating current, i_(max), for the one or more TEMs, and morepreferably less than 15% of i_(max). For example, if using a TEM withan, i_(max) of 3.0 A a suitable TOC may be In the range of 200-600 mA,such as in the range of 250-500 mA, or in the range of 250-400 mA, suchas about 300 or 350 mA.

FIGURE LEGENDS

FIG. 1 illustrates schematically an embodiment of the invention, Thedevice shown comprises an LED (10), a chamber (1), a first TEM (3)connected to the LED with a heat slug (11) of a thermally conductingmaterial, two additional TEMs (5,6) and thermally conducting plates (4)connecting the first and second TEMs (3,5) and the second and third TEM(5,6).

FIG. 2 is a graph showing fitted curves based on Equations Ia and Ib fortwo 1 W LED diodes; amber (triangles) and red (squares), normalizedaround 25° C.

FIG. 3 shows the cooling power (diamonds) and Joule heat (squares) asfunctions of the operating current in Amperes, in a TEC of the followingparameters: R=2 Ohm, K=0,6 W/K, α=0,05 V/K, T_(G)=480K, (T_(I)/I)=40K/A.

FIG. 4 shows a blow-up of the low-current regime of the diagram of FIG.3.

FIG. 5 shows the coefficient of performance (COP) as a function of theoperating current in the low-current regime of a TEM having the samespecifications as described listed above in the legend for FIG. 3.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The preferred embodiment described herein is Illustrated schematicallyin FIG. 1 and comprises an LED (10) based fixture with heat transferapparatus attached to the LEDs, mounted individually and or in groups incritical outdoor applications for the purpose of light signalling and orwide area illumination type applications. The apparatus consists of aclosed chamber (1) insulated with aerogel type material (2) to preventambient heat from loading the total heat removal. Inside the chamber isa thermo-electric module (3) with the hot side facing a thermallyconductive plate (4). Said plate is the outmost boundary between thechamber and the adjacent micro-environment. Between the TEM (3) and theLED (10) is a conical heat slug (11) covering the LED thermal spot onthe top and in contact of the cold side surface of the TEM on the otherside. The total contact area for the LED (10) is less than the totalarea of the TEM (3). Outside the chamber (1) the cold side of a secondTEM (5) is mounted on the thermally conductive plate (4) while a similarplate (4) is placed between TEM (5) and TEM (6). TEM (6) is the lastunit next to the heat sink (7). The number of sets of TEMs and thermallyconductive plates (4) placed In concession can vary. The heat sink (7)is constructed according to presented formulas. It Is connected to anexternal structure (8) for final removal of the total heat.

If the LEDs are arranged within a housing the thermoelectric module mayalso be attached to the housing, and/or be a structural component ofsaid housing.

The LEDs can be grouped in any geometrical order and attached to anycurved and/or even surface. The angularity and alignment of LEDs (10)for the purpose of illumination and signalling is not an issue in thisinvention. The Peltier thermoelectric module (3) has one side hot andthe other cold when activated with an electric current. The cold side isfacing the connecting plate (11) attached to the LED (10) or could bedirectly mounted on the LED. A thermally conducting metal plate (4) isattached to the hot side of the TEM (3). Insulating material (2)(preferably aerogel) is fit around components to prevent heat flow fromthe ambient environment to the cooling area when ambient temperature ishigher than the operating temperature of the cold side of the TEM(3,5,6) and to prevent back flow of heat in the system. Aerogels (2)have a very low thermal conductivity and even in very thin layers theyare capable of insulating and or stopping the heat flow. The transitionbetween the hot side of the TEM (3) and the heat sink (4) must be avapour free material and able to withstand 1 bar pressure. The describedembodiment can be constructed having two chambers. Chamber 1(A) is forthe LEDs and the TEM. It is Insulated with aerogels (2) except where thethermally conducting plate (4) is attached. The chamber (1) can befilled with dry air or other gases (inert gases) to a higher airpressure than average ambient pressure to prevent the flow of gases (inparticular ambient air carrying moisture) into the chamber. The chamber(1) can be filled with gases other than air, e.g. Nitrogen, Argon orHelium, to further prevent moisture inside the chamber. A second chambercan be constructed surrounding the space (B) to ensure more efficientmovement of heat from the thermally conducting plate (4) to the finalheat sink (7)—and then to the support structure (8). Using another TEM(5) or (5,6) cascading modules (5,6) enables stabilization of the heatflow and provides a more constant temperature around the operating LED.

1. A light illuminating device comprising: a. at least one lightemitting diode (LED), b. at least one thermoelectric module (TEM) havinga first surface which is thermally connected to the LED, c. a heat sinkthermally connected to a second surface of the at least one TEM, d. athermally insulating cover creating an enclosed chamber substantiallyinsulating the LED from ambient air.
 2. The device of claim 1, whereinthe at least one TEM is configured such that the device is operated byrunning a TEM-operating current (TOC) through the TEM, which current isless than 20% of the maximum operating current for the TEM, therebypreventing a decrease in light output due to an increase of thetemperature of the LED(s).
 3. The device of claim 2, wherein the atleast one TEM is configured such that the device is operated by runninga TOC through the TEM, which current is less than 15% of the maximumoperating current for the TEM.
 4. The device of claim 1, wherein the TEMIs configured such that the operating temperature of the LED(s) is lowerthan or about the same as the ambient temperature surrounding thedevice.
 5. The device of claim 1, wherein the TOC for each of said atleast one TEM is in the range of 200-600 mA.
 6. The device of claim 5,wherein the TOC for each of said at least one TEM is in the range of250-500 mA.
 7. The device of claim 1, wherein the TEM has a coefficientof performance (COP) during normal operation in the range of about 2-6.8. The device of claim 1, which device is configured such that thedevice produces more illumination per unit consumed power when the TOCis applied to the TEM, than the illumination produced per unit consumedpower when no TOC is applied to the TEM.
 9. The device of claim 1,comprising a plurality of LEDs.
 10. The device of claim 1, comprising aplurality of TEMS.
 11. The device of claim 1, comprising a plurality ofTEMs thermally connected in a stacked fashion.
 12. The device of claim1, wherein the enclosed chamber has a higher pressure than ambientpressure during normal operation.
 13. The device of claim 1, wherein theenclosed chamber has a lower pressure than ambient pressure, duringnormal operation.
 14. The device of claim 1, further comprising acontrol unit for controlling the TOC, and one or more sensors connectedto the control unit for sensing one or more environmental parameters,wherein the control unit is configured to adjust the TOC based onparameters measured by the one or more sensors.
 15. The device of claim14, wherein said one or more sensors comprise a temperature sensor formeasuring the in situ temperature surrounding the LED(s).
 16. The deviceof claim 14, wherein said one or more sensors comprise a sensor formeasuring emitted light from the LED(s)
 17. The device of claim 14,which is operated with pulsed current to the one or more LEDs.
 18. Thedevice of claim 1 which device is configured for an application selectedfrom traffic light, illuminated roadway and/or emergency signs, airportrunway lights, vehicle lights Including brake lights.
 19. A lightilluminating device comprising at least one light emitting diode (LED)and at least one thermoelectric module (TEM) thermally connected to theLED, and a heat sink; wherein the at least one TEM is selected andconfigured such that by running a TEM-operating current (TOC) throughthe TEM, the thermal power produced by the at least one LED istransferred through the at least one TEM to the heat sink, therebymaintaining or lowering the temperature surrounding the LED andenhancing the light output from the LED; the device thus consuming lessoverall power per amount of emitted light when the TEM is running ascompared to the overall power per same amount of light when the deviceis operated without running an operating current through the TEM.
 20. Amethod for enhancing the efficiency of an light illuminating devicehaving one or more LEDs as a light source, comprising: a. providing thedevice with one or more thermoelectric module(s) (TEM) having a coldsurface and a hot surface, such that the cold surface is thermallyconnected to the LED and the hot surface is thermally connected to aheat sink, b. applying a TEM-operating current (TOC) to the one or moreTEMs to create a temperature gradient through the TEM, c. adjusting theTOC such that substantially all of the thermal energy created by theLED(s) when operated is transferred to the heat sink, therebysubstantially maintaining the operating temperature of the LED(s) atambient temperature or a lower temperature, d. wherein the TEM isconfigured and TOC adjusted such that the device consumes less overallpower per amount of emitted light when the TEM is running as compared tothe overall power per same amount of light when the device is operatedwithout applying a TOC to the TEM.
 21. The method of claim 20, whereinthe TOC is less than 20% of the maximum operating current for the one ormore TEMs.
 22. The method of claim 20, wherein the TOC is less than 15%of the maximum operating current for the one or more TEMs.
 23. Themethod of claim 20, wherein the TOC for each of said at least one TEM isin the range of 200-600 mA.
 24. The method of claim 20, wherein the TOCfor each of said at least one TEM is in the range of 250-500 mA.